The present invention relates to a novel process for producing silicon isocyanate compounds. More specifically, the present invention relates to a novel process for producing silicon isocyanate compounds which comprises reacting a silicon halide compound with an alkali salt of isocyanic acid or cyanic acid in the presence of a reaction accelerator.
A silicon isocyanate compound readily reacts with a compound containing a hydroxyl group or amino group to form a silyl urea linkage or siloxane linkage. Therefore, the silicon isocyanate compound is useful as an intermediate or a raw material for various organosilicon compounds or a material for various compositions.
The term "silicon isocyanate compound" as used herein refers to a compound containing one or more Si--NCO linkages. Such a compound has been hitherto prepared by reacting a silicon compound containing a Si--X linkage, wherein X represents a halogen atom, with silver isocyanate or lead cyanate. However, such a conventional process provides a poor yield and is disadvantageous from an economical point of view because an expensive silver or lead salt is used as a starting material of the reaction. For this reason, it would be advantageous if salts of alkali metals such as sodium and potassium, and alkaline earth metals such as barium or ammonium, which are relatively easily available, could be used in place of the silver or lead salt. These metallic salts, however, are not satisfactory with respect to reactivity. Therefore, these metallic salts have been considered to be unsuitable for use in the production of a silicon isocyanate compound.
DOS No. 1,965,741 (published July 8, 1971) discloses that a silicon isocyanate compound can be synthesized by reacting an alkali metal cyanate or ammonium isocyanate with a silicon halide in the presence of a certain reaction accelerator. As the reaction accelerator, a specially selected compound having a dielectric constant of 10 or more, such as acid amides and N-methylpyrolidone is used. Also, the process disclosed in the above-mentioned patent publication uses an organic solvent having a relatively high boiling point such as cyclohexanone and benzonitrile. The use of such a solvent is inconvenient in view of the reaction operation. Therefore, the process is not advantageous for practical purposes.
In view of the above described state of the art, we have made studies to obtain a silicon isocyanate compound containing Si--NCO linkage(s) from the reaction of an alkali metal, alkaline earth metal or ammonium salt of cyanic acid or isocyanic acid, which salt is easily available, with a silicon halide compound in a higher yield at a lower cost as compared with the conventional process. As a result, we have discovered that the use of a reaction accelerator selected from alkyl amines, nitroalkanes and crown ethers can produce a silicon isocyanate compound in a high yield while the amount of the reaction accelerator added is reduced to a low level. Thus, the present inventors have applied for a patent (Japanese Patent Application No. 101647/1979).
In accordance with the above mentioned process, a silicon isocyanate compound can be produced from a silicon halide compound and an isocyanate or cyanate salt, which is inexpensive and has low toxicity. Therefore, the process is considered to be advantageous as compared with the conventional process. However, there is room for further improvement in the process, because alkyl amines, nitroalkanes and crown ethers, which are used as the reaction accelerator, require special handling, and the product is difficult to separate and refine from the reaction mixture.